The world has never been more technologically advanced than it is now, but that doesn’t mean that some things haven’t been lost along the way. Many of the technologies, inventions, and manufacturing processes of antiquity have simply disappeared with the passage of time, while others are still not fully understood by modern day scientists. Some have since been rediscovered (indoor plumbing, road building), but many of the more mysterious lost technologies have gone on to become the stuff of legend. Here are ten famous examples:

10. Stradivari Violins

One lost technology of the 1700s is the process through which the famed Stradivari violins and other stringed instruments were built. The violins, along with assorted violas, cellos, and guitars, were constructed by the Stradivari family in Italy from roughly 1650-1750. The violins were prized in their day, but they’ve since become world famous for having an unparalleled—and impossible to reproduce—sound quality. Today there are only around 600 of the instruments left, and most are worth several hundred thousand dollars. In fact, the name Stradivari has become so synonymous with quality that it has come to serve as a descriptive term for anything considered to be the best in its field.

Lost?

The technique for building Stradivari instruments was a family secret known only by patriarch Antonio Stradivari and his sons, Omobono and Francesco. Once they died, the process died with them, but this hasn’t stopped some from trying to reproduce it. Researchers have studied everything from fungi in the wood that was used to the unique shaping of the bodies in order to describe the famous resonance achieved by the Stradivarius collection. The leading hypothesis seems to be that the density of the particular wood used accounts for the sound. Still, some dispute the claim that the instruments are special at all. In fact, at least one study concluded that most people don’t even notice a difference in sound quality between a Stradivari violin and a modern counterpart.

9. Nepenthe

The sheer sophistication of the technology wielded by the ancient Greeks and Romans is often quite astonishing, especially when it came to medicine. Among other things, the Greeks were known to treat the bereaved with Nepenthe, a primitive anti-depressant that was known for its ability to “chase away sorrow.” The drug is frequently mentioned in Greek literature like Homer’s Odyssey. Some claim that it might be fictional, but others have argued that the drug was real and used widely in ancient Greece. Nepenthe was said to have originated in Egypt, and its effects as “a drug of forgetfulness” have led many to compare it to opium or laudanum.

Lost?

Oftentimes these “lost” technologies are possibly still around today, and it’s only our inability to identify their modern equivalent that makes them mysterious. Supposing that it really did exist, this is probably the case with Nepenthe. The drug is most likely still used today, but historians are unable to pinpoint just what modern substance the Greeks were referring to. Opium is definitely the most popular choice, but other frontrunners include wormwood extract and scopolamine.

8. The Antikythera Mechanism

One of the most mysterious of all archeological artifacts is what is known as the Antikythera Mechanism, a bronze machine that was discovered by divers off the coast of the Greek island of Antikythera in the early 1900s. The mechanism consists of a series of over 30 gears, cranks, and dials that could be manipulated in order to chart the astronomical positions of the sun, moon, and other planets. The device was found among the remains of a shipwreck that scientists have dated to the 1st or 2nd century BC. Its true purpose is still not fully known, and the mystery behind its construction and use has puzzled researchers for years. The consensus now seems to be that the Antikythera Mechanism was a kind of primitive clock that could calculate lunar phases and solar years, which has led some to refer to it as the earliest example of an “analog computer.”

Lost?

The sophistication and precision evident in the design of the mechanism suggests that it was not the only device of its kind, and many scientists have speculated that its use might have been widespread. Still, the existence of other devices like the Antikythera Mechanism doesn’t appear on the historical record until the 14th century, which would mean that the technology was lost for nearly 1400 years. Why or how will probably remain a mystery, especially since the mechanism still stands as the only ancient discovery of its kind.

7. The Telharmonium

Often recognized as the world’s first electronic musical instrument, the Telharmonium was a large organ-like device that used tonewheels to creative synthetic musical notes that were then transmitted by wires to a series of loudspeakers. The Telharmonium was developed by the inventor Thaddeus Cahill in 1897, and at the time it was one of the biggest instruments ever built. Cahill would eventually construct three versions of it, one of which was said to weigh some 200 tons and take up enough space to fill an entire room. Its set up consisted of a collection of keyboards and foot pedals, which the user could manipulate to reproduce the sounds of other instruments, particularly woodwinds like flutes, bassoons, and clarinets. The first public exhibitions of the Telharmonium were met with great success. People came in droves to hear public performances of the primitive synthesizer, which was said to produce a clear, round sound that resembled a sine wave.

Lost?

Following its initial successes, Cahill developed big plans for his Telharmonium. Because of its ability to transmit a signal over telephone wires, he envisioned Telharmonium music being broadcast remotely as background sound in places like restaurants, hotels, and private homes. Unfortunately, the device proved to be too far ahead of its time. Its massive energy consumption strained early power grids, and at a price tag of a whopping $200,000, the instrument was just too pricey to build on a large scale. What’s more, early experiments in broadcasting its music over the telephone proved disastrous, as its sound would often bleed over into private phone conversations. After a while, the public’s fascination with the device waned, and the different versions of it were eventually scrapped. Today, nothing remains of the original three Telharmoniums—not even sound recordings.

6. The Library of Alexandria

Although it wasn’t a technology, the legendary Library of Alexandria warrants a place on this list, if only because its destruction meant that so much of the collected knowledge of antiquity was forever lost. The library was founded in Alexandria, Egypt in roughly 300 B.C., most likely during the reign of Ptolemy Soter. It marked the first serious attempt to gather all the known information about the outside world in one place. The size of its collection is not known (though the number has been estimated to be in the neighborhood of one million scrolls), but the library undoubtedly attracted some of the great minds of its day, among them Zenodotus and Aristophones of Byzantium, both of whom spent considerable time doing scholarly work in Alexandria. The library became so important that there is even a legend that all visitors to the city would have to surrender their books upon entering so that a copy could be made for storage in the great library.

Lost?

The Library of Alexandria and all its contents burned sometime around the first or second century AD. Scholars are still uncertain just how the fire was started, but there are a few competing theories. The first, which is backed up by historical documents, suggests that Julius Caesar accidentally burned the library when he set fire to some of his own ships in order to block the path of an advancing enemy fleet. The fire spread to the docks and then enveloped the library. Other theories contend that the library was sacked and burned by invaders, with the Emperor Aurelian, Theodosius I, and the Arab conqueror Amr ibn al ‘Aas serving as the main contenders. However the Library of Alexandria was destroyed, there’s little doubt that many of the secrets of antiquity were lost along with it. We’ll never know for sure just what was lost, but had it remained standing, there’s an argument to be made that many of the technologies on the list would have never been lost.

5. Damascus Steel

Damascus steel was an impossibly strong type of metal that was widely used in the Middle East from 1100-1700 AD. It is most famously associated with swords and knives. Blades forged with Damascus steel were known for their amazing strength and cutting ability, and were said to be able to slice rocks and other metals—including the blades of weaker swords—cleanly in half. The blades are believed to have been created using wootz steel, which was most likely imported from India and Sri Lanka and molded and blended to create a patterned blade. The special quality of the swords is thought to have derived from this process, which weaved together tough cementite and soft iron to form a metal that was as strong as it was flexible.

Lost?

The particular process for forging Damascus steel appears to have disappeared sometime around 1750 AD. The exact cause for the loss of the technique is unknown, but there are several theories. The most popular is that the supply of ores needed for the special recipe for Damascus steel started running low, and sword makers were forced to develop other techniques. Another is that the whole recipe for Damascus steel—specifically the presence of carbon nanotubes—was only discovered by accident, and that sword smiths didn’t actually know the technique by heart. Instead, they would simply forge the swords en masse, and test them to determine which met the standards of Damascus steel. Whatever the technique, Damascus steel is one technology that modern experimenters have been unable to fully reproduce. There are pattern welded knives that are marketed as being made from “Damascened steel”, but while usually well made, they are only approximations of the lost technique for real Damascus steel.

4. Apollo/Gemini Space Program Technology

Not all lost technology dates back to antiquity—sometimes it’s just become so obsolete that it’s no longer compatible. The Apollo and Gemini space programs of the 50s, 60s, and 70s were responsible for NASA’s biggest successes, including some of the first manned space flights and the first trip to the moon. Gemini, which ran from 1965-66, was responsible for the much of the early research and development into the mechanics of human space flight. Apollo, which followed shortly thereafter, was launched with the goal of landing a crew on the surface of the moon, which it succeeded in doing in July of 1969.

Lost?

The Apollo and Gemini programs aren’t truly lost. There are still one or two Saturn V rockets lying around, and there are plenty of parts from the spacecraft capsules still available. But just because modern scientists have the parts doesn’t mean they have the knowledge to understand how or why they worked the way they did. In fact, very few schematics or records from the original programs are still around. This lack of record keeping is a byproduct of the frenetic pace at which the American space program progressed. Because NASA was in a space race with the USSR, the planning, design, and building process of the Apollo and Gemini programs was always rushed. Not only that, but in most cases private contractors were brought in to work on every individual part of the spacecraft. Once the programs ended, these engineers—along with all their records—moved on. None of this would be a problem, but now that NASA is planning a return trip to the moon, a lot of the information about how the engineers of the 1960s made the voyages work is invaluable. Amazingly, the records remain so disorganized and incomplete that NASA has resorted to reverse engineering existing spacecraft parts that they have lying around in junkyards as a way of understanding just how the Gemini and Apollo programs managed to work so well.

3. Silphium

Lost technologies aren’t always the result of too much secrecy or poor record keeping—sometimes nature just doesn’t cooperate. This was the case with Silphium, an herbal wonder drug that the Romans used as one of the earliest forms of birth control. It was based on the fruit of a particular genus of the fennel plant, a flowering herb that only grew along a certain shoreline in modern day Libya. The heart-shaped fruit of the Silphium plant was known to be something of a cure-all, and was used to treat warts, fever, indigestion and a whole host of other ailments. But it was Silphium’s powers as a contraceptive that made it one of the most valuable substances in the Roman world, to the point that the plant appears on several different pieces of ancient Roman currency. Women would drink Silphium juice every few weeks, and this would be enough to prevent pregnancy. Using the herb would even terminate an existing pregnancy if used correctly, which would make Silphium one of the earliest methods of abortion.

Lost?

Silphium was one of the most sought after drugs of the ancient world, and its use spread rapidly across Europe and into Asia. But despite its remarkable effects, the particular genus of plant needed would only grow in one area along the Mediterranean in North Africa. Its scarcity, combined with an overwhelming demand, more than likely led to over harvesting, which drove the plant into extinction. Because the particular species no longer exists, modern scientists are unable to examine Silphium to see if its powers of contraception were really as effective as Roman historians and poets would lead one to believe, or if there were any adverse side effects. Still, it is worth noting that other herbs that are chemically similar to Silphium have been proven to have a fairly high rate of preventing pregnancy.

2. Roman Cement

Modern concrete was developed in the 1700s, and today the simple mixture of cement, water, sand, and rocks is the most widely used building material in the world. But the recipe developed in the 18th century wasn’t the first time concrete was invented. In fact, concrete was widely used throughout antiquity by the Persians, Egyptians, Assyrians, and Romans. The Romans in particular made extensive use of concrete, and they were responsible for first perfecting the recipe by mixing burnt lime with crushed rocks and water. Their mastery of its use allowed them to build many of their most famous structures, among them the Pantheon, the Colosseum, the aqueducts, and the Roman Baths.

Lost?

Like so many technologies of the Greeks and Romans, the recipe for concrete was lost during the descent into the Dark Ages, but just why remains a mystery. The most popular theory is that the recipe was something of a trade secret among stonemasons, and that the method for making cement and concrete died along with those who knew it. Perhaps even more interesting than the disappearance of Roman cement are the particular qualities that separate it from more modern Portland cement, which is the most common type of cement used today. Structures built with Roman cement, like the Colosseum, have managed to weather thousands of years of punishment from the elements and remain standing, but buildings constructed with Portland cement have been known to wear down much faster. This has been theorized to be the result of different chemicals that the Romans added to their cement, among them milk and even blood. These were said to create air bubbles within the concrete that helped the material to expand and contract in the heat and cold without damaging itself.

1. Greek Fire

Perhaps the most famous of all lost technologies is what is known as Greek Fire, an incendiary weapon that was used by the military of the Byzantine Empire. A primitive form of napalm, Greek Fire was a kind of “sticky fire” that would continue burning even in water. The Byzantines most famously used it during the 11th century, when it was credited with helping to repel two sieges of Constantinople by Arab invaders. Greek Fire could be deployed in many different ways. In its earliest form it was poured into jars and thrown at enemies like a grenade or a Molotov cocktail. Later, giant bronze tubes were mounted on warships, and siphons were used spray the weapon at enemy vessels. There was even a kind of portable siphon that could be operated by hand in the style of a modern flamethrower.

Lost?

The technology behind Greek Fire certainly isn’t completely alien. After all, modern militaries have now been using similar weapons for years. Still, the closest counterpart to Greek Fire, napalm, wasn’t perfected until the early 1940s, which would mean the technology was lost for several hundred years. The weapon’s use seems to disappear after the decline of the Byzantine Empire, but just why still isn’t known. Meanwhile, the possible chemical composition of Greek Fire has been widely studied by historians and scientists. An early theory was that the mixture included a heavy dose of saltpeter, which would make it chemically similar to gunpowder. This idea has since been rejected, because saltpeter wouldn’t burn in water. Instead, modern theories propose that the weapon was more likely a cocktail of petroleum and other chemicals, possibly including quicklime, niter, or sulfur.